FIG. 1 shows a device DEV comprising an electrical circuit module MO and at least one electrical component CO. The electrical circuit module MO comprises a base 1 and a metallized ground plane 2 which is for example made of aluminium or copper. The electrical component CO also comprises a base 3 and a metallized ground plane 4. The two ground planes 2, 4 are to be brought into contact with each other, as indicated with the arrow in FIG. 1. If for example the electrical component is a patch antenna, the ground plane 2 should normally have a size 3-times of the size of the ground plane 4 of the electrical component CO to achieve an effective grounding. The portion 5 can comprise other circuit components 6 which are interconnected via a conductor pattern 7.
It should be noted that the electrical component CO can in principle be any component which requires the connection to a ground plane as for example provided by the electrical circuit module MO. Such grounding problems often occur in high frequency circuits, for example when the electrical circuit module is a high frequency circuit. An example for the electrical component CO is a micro-strip patch antenna, in particular a ceramic patch antenna.
FIG. 3 shows the principle structure of such a ceramic patch antenna as generally known in the art and described in "CAD of micro-strip antennas for wireless applications" by Robert A. Sainati, Artech House, INC., 1996, ISBN 0-89006-562-4, pages 1-5, 18, 19 and 50, 51. FIG. 3a shows a cross sectional view of such a ceramic patch antenna, FIG. 3b is a bottom view and FIG. 3c is a perspective view. Ordinary patch antennas consist of rectangular or round slices of a ceramic material 3 with a preferrably high dielectric constant .di-elect cons..sub.r. Furthermore, a conducting layer pattern 11 is provided on the upper surface and a ground plane 4 is provided on the lower surface. Typically, the conducting layer pattern 11 is rectangluar or squared in shape or consists of a pattern of single stripes depending on the desired radiation pattern.
In order to feed energy to the conducting layer 11, the ground plane 4 has an opening 13 through which a feeding pin 12 extends. The feeding pin 12 can be soldered to the conducting layer pattern 11. Therefore, when the component CO is constituted by such a ceramic patch antenna as is shown in FIG. 3, when mounting the component CO to the electrical circuit module MO, also the circuit module MO (cf. FIG. 1), i.e. the circuit board, has a corresponding opening in its ground plane 2 in order to avoid a short circuit. Energy is then fed to the feeding pin from a lower layer of the multilayer board MO.
In a mobile telephone such a ceramic patch antenna is for example used as part of a GPS receiver. Therefore, when mounting such a component CO onto the electrical circuit module MO, the available space must be used as efficiently as possible and furthermore, from a manufacturing point of view, an easy mounting technique should be used. Generally, the antenna characteristic is mainly determined by the mechanical dimensions of the conducting layer 11 and the substrate 3 as well as the substrate properties.
Regarding the efficient use of the available space, substrates 3 with a high dielectric constant .di-elect cons..sub.r lead to reductions in size, because the effective wavelength .lambda..sub.e diminishes according to the equation .lambda..sub.e =.lambda..sub.O /.di-elect cons..sub.r +L (.lambda..sub.0 =free space wavelength, .di-elect cons..sub.r +L =shortage factor). Thus, with increasing dielectric constant .di-elect cons..sub.r, the effective wavelength .lambda..sub.e decreases. For decreasing values of .lambda..sub.e, the area of the conducting layer 11 can in general be reduced. On the other hand, for a proper operation, a ceramic patch antenna requires a ground plane 2 in the module 110 which should be around 3-times larger than the ceramic substrate 3.
As shown in FIG. 2, typically the circuit module MO is constituted by a multilayer board (here a 6 layer board) where conductor patterns 5.sub.1, 5.sub.2, 5.sub.3 may be provided on several levels, possibly with intermediate insulation layers 10.sub.1, 10.sub.2. In such multilayer board configurations, generally through-holes 8 as well as blind-holes 9.sub.1, 9.sub.2 are used. The through-holes 8 may connect all conductor patterns of all levels by means of a metallization which is applied to the inside walls of the hole 8. On the other hand, blind holes 9 only connect two or more layers without extending through all layers. It is clear from FIG. 2 that in the conventional configuration, there is no possibility to use the large area (called the restricted area in FIG. 2) directly underneath the ceramic patch antenna for through-holes or routing purposes for signals not having a ground potential, since all signals would invariably be short-circuited to the ground plane 4 of the electrical component CO, if the through-hole would be placed in the restricted area (the inside metallization in the hole 8 would directly merge into the ground plane pattern. That is, no conductor pattern can be used for the routing of other signals of non-ground potential. This leads to an increase in size of the device, since the through-holes must be placed at other parts of the circuit module. It should be noted that FIG. 2 shows the device before all layers are fitted together, i.e. the spaces indicated with {character pullout} in FIG. 2 are spaces which collapse once all layers are fitted together under high pressure/high tension.
Furthermore, two techniques have been used up to now in order to mount the component CO to the circuit module MO such that a good contact between the two ground planes 4, 2 is maintained even in the presence of vibrations or shocks. One conventionally used method is to provide an adhesive material on one of the ground planes 4, 2 and to then stick the two parts together. However, the adhesive material must be conductive and may reduce the optimal contacting of the two ground planes 4, 2 by introducing additional resistances. Furthermore, the adhesive may deteriorate over time and thus the adhesion/cohesion strength may also decrease. For example, when a patch antenna is used as part of a GPS receiver, even small deviations or deteriorations may lead to a reduced strength of the received signal and hence to a degradation in the position estimation performance.
Therefore, another conventionally used method is reflow soldering where a soldering paste is applied to one ground plane or to both ground planes 2, 4. The component CO is e.g. placed on the ground plane of the circuit module MO and hot air or an infrared radiation is applied to solder the component CO onto the circuit module MO. However, it is quite difficult to reflow solder such ceramic patch antennas, because major parts of the area that needs to be soldered are covered by the component. Vaporized solder fluid as a result of the application of the hot air or the infrared radiation can not escape from the area underneath the component and can lead to bubbles (i.e. small cavities) between the ground plane 2 and the ground plane 4. This can drastically reduce the electrical conducting properties.
Furthermore, if portions or even the complete area of the grounding plane 4 is soldered, such large area solder joints can lead to tensions between the component CO and the circuit module MO, since most likely the component CO and the circuit module MO have different temperature coefficients (e.g. the ceramic substrate 3 and the substrate 1 of the component CO and the circuit module MO will most certainly have different temperature coefficients). This can result in an undesired bending and in twisting of the board and may also reduce the conducting properties since due to the deformations some parts of the two ground planes 2, 4 may not be in contact with each other. Furthermore, the deformations caused by the application of the comparatively high temperature during the soldering process may cause a breakage of the connection between portions of the ground planes 2, 4 or the breakage of internal connections in the circuit module, i.e. the multilayer board, over time.
The U.S. Pat. No. 5,517,162 describes a dielectric resonator device including a dielectric member having an outer surface, a plurality of inner conductors provided in the dielectric member, an outer conductor disposed on the outer surface of said dielectric member and having a mounting surface, signal input and output electrodes disposed on the outer surface of said dielectric member to oppose a mounting substrate and electrically coupled with said inner conductors and a plurality of solder bumps disposed on both the mounting surface of the outer conductor and on said signal input and output electrodes. The plurality of solder bumps are located so as to mechanically and electrically connect said outer conductor and said signal input and output electrodes to the mounting substrate when the solders are melted. This prior document only discloses a pattern of solder bumps for two different pontentials such that during the melting process a connection of the electrodes is avoided.
The patent abstracts of Japan, vol. 98, no. 9, Jul. 31, 1998 & JP 10 093474 A discloses an antenna multicoupler. To maintain a grounded potential of high frequency band in a wide range and to obtain high performance characteristics a solder bump is formed at the rear side of a multilayer substrate of an antenna multilayer to maintain a fixed interval from a grounded surface of a mounted substrate.
In both afore-mentioned documents it is only disclosed that solder bumps according to a predetermined arrangement pattern are provided and the solder bumps melt during the soldering process. Thus, such kind of solder bumps cannot be regarded as the spacer elements of the connecting means according to the present invention as will be explained below.
U.S. Pat. No. 5,635,942 describes a special patch antenna and shows in FIG. 3 a device including an electrical component and an electrical circuit module both having flat ground plains.
The European patent application EP 0 588 465 A1 describes a microstrip antenna and shows in FIG. 1 a similar construction as explained with reference to FIG. 3 in the present application. That is, an electrical component and an electrical circuit module both have flat ground plains.